Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
Control of the position of the heads is typically achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM, issued to Duffy et al., assigned to the assignee of the present invention. A typical servo system utilizes servo information that is written to the discs during the disc drive manufacturing process to detect and control the position of the heads through the generation of a position error signal (PES) which is indicative of the position of the head with respect to a selected track. More particularly, during track following in which the head is caused to follow a selected track, the servo system generates the PES from the received servo information and then uses the PES to generate a correction signal which is provided to a power amplifier to control the amount of current through the actuator coil, in order to adjust the position of the head accordingly.
Typically, the PES is presented as a position dependent signal having a magnitude indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Thus, it is common for the PES to have normalized values corresponding to a range of, for example -1.0 to +1.0, as the head is swept across a selected track and to have a value corresponding to a value of 0 when the head is positioned over the center of the track. As will be recognized, modern servo systems typically generate the PES as a sequence of digital samples which generally correspond to the above analog range.
The PES is generated by the servo system by comparing the relative signal strengths of burst signals generated from precisely located magnetized servo fields in the servo information on the disc surface. The servo fields are generally arranged in an "offset checkerboard" pattern so that, through manipulation of the magnitudes of the burst signals provided to the servo system as the servo fields are read, the relative position of the head to a particular track center can be determined and controlled. More particularly, digital representations of the analog burst signals are typically provided to a servo loop microprocessor (or digital signal processor), which obtains a digital representation of the value of the PES from a selected combination of the input digital representations of the analog burst signals. The microprocessor then compares the value of the PES to a desired value indicative of the desired position of the head to the selected track and issues a digital correction signal to the power amplifier, which in turn provides an analog current to the actuator coil to adjust the position of the actuator accordingly.
The servo information, including the servo fields, are written to the discs during the manufacturing process using a highly precise servo track writer. Although methodologies vary in the writing of the servo information, typically the disc drive is mounted on the servo track writer and the appropriate write signals are provided to the heads of the disc drive to write the servo information while the discs are rotated by the disc drive spindle motor. A mechanical pusher arm is used to incrementally advance the heads over the surfaces of the discs while a closed loop positional control system ensures the heads are properly located relative to the discs. Depending upon a particular configuration, each servo field is typically written using a plurality of rotations of the disc, with a portion of the servo field being written during each rotation of the disc.
Although servo track writers have proven to be highly precise and reliable (sufficient to support disc drive data storage areal densities exceeding 1 Gbit/in.sup.2), errors have been found to occasionally occur during servo track writing operations. For example, it is common for a servo track writer to write a selected servo field using a sinusoidal write signal of selected magnitude and phase over a plurality of passes of the head so that a portion of the field is written during each pass. If during one of the passes the servo track writer erroneously uses a sinusoidal write signal that is out of phase, the resulting servo field, though precisely located on the disc, will produce a burst signal having a reduced magnitude. Because the servo system relies upon the relative magnitudes of the servo fields, such reduction in magnitude can adversely affect the ability of the servo system to discern the location of the head with respect to the track and control the position of the head.
Moreover, localized anomalies in the media can prevent the generation of burst signals having the proper relative magnitudes, even when the servo fields have been otherwise properly formed during the servo track write operation.
Accordingly, there is a need for an improved approach to detecting the defective servo frames including anomalous position field patterns which generate burst signals of insufficient relative magnitude to facilitate proper servo control.